1. Field of the Invention
This invention relates to a liquid crystal display device of an MVA mode that controls an alignment direction of liquid crystal molecules by utilizing protrusive structures disposed on a substrate surface or non-electrode regions (slits) of each transparent pixel electrode, and a liquid crystal display device that regulates an alignment direction of liquid crystal molecules by using a photo-cured product formed of a photo-curing composition having a liquid crystal skeletal structure.
2. Description of the Related Art
A liquid crystal display device has been utilized not only for mobile application such as notebook type personal computers and portable information terminals but also for desktop type computers and television monitors to satisfy social requirements for space saving and energy saving. Performance of the liquid crystal display device has been drawn into comparison with that of CRT monitors. Development has been made to improve performance of the liquid crystal display device to the level of performance of the CRT monitors.
The CRT monitor makes use of light emission of a fluorescent material and has low angle dependence. Its viewing angle is wide. On the other hand, the liquid crystal display device electrically controls alignment directions of the liquid crystal molecules when the rays of light from a light source transmit through a liquid crystal layer to change a transmission and to display images. Even though contrast in a vertical direction to a screen can be satisfied, the liquid crystal display device has large viewing angle dependence such that contrast is small when viewed from an oblique direction and chromatic inversion develops.
For example, a liquid crystal display device of a twisted nematic (TN) mode, in which molecules of a liquid crystal material having positive dielectric anisotropy is arranged horizontally to a substrate surface and is twisted 90 degrees between opposing substrates, has been widely utilized as a liquid crystal display (LCD) using an active matrix. The greatest drawback of this TN mode LCD is that the viewing angle is small. Various studies have therefore been made to improve this viewing angle characteristic.
For example, an MVA (Multi-domain Vertical Alignment) mode is known that divides one pixel into multiple domains and disperses the alignment direction for each domain to enlarge the viewing angle of one pixel as a whole.
FIG. 3 is a conceptual view of the MVA mode liquid crystal display device. Reference numerals 21 and 22 denote upper and lower glass substrates. Reference numerals 61 and 62 denote protrusive structures formed on these substrates, respectively. Reference numeral 5 denotes liquid crystal molecules. Besides these members, the liquid crystal display further includes transparent electrodes, alignment films, thin film transistors (TFT), phase difference films, and the like, that are not shown in the drawing.
When a liquid crystal material having negative dielectric anisotropy is sealed between two glass substrates, the liquid crystal molecules are aligned substantially vertically by the force of regulation of the alignment film. Pixel electrodes connected to TFT are formed on one of the glass substrates, and a common electrode is formed on the other glass substrate. The protrusive structures are alternately arranged on the pixel electrodes and on the common electrode.
When TFT is OFF state, the liquid crystal molecules in the gap region between the protrusive structures is aligned in the substantial vertical direction to the interfaces of the substrates, and the liquid crystal molecules in the proximity of each protrusive structure are aligned in the direction vertical to a slope.
When TFT is turned OFF state, an electric field is applied to the liquid crystal, and the liquid crystal molecules are about to turn down from the direction vertical to the substrate. Due to the inclination of the protrusive structures, however, the liquid crystal molecules are regulated to the turn-down direction of the liquid crystal molecules that have turned down from the beginning, so that inclination propagates to the liquid crystal molecules in the gap region.
FIG. 4 shows the inclining direction of the liquid crystal molecules inside one pixel of the MVA mode liquid crystal display device. Reference numerals 61 and 62 denote the protrusive structures disposed on the upper and lower substrates. Reference numeral 7 denotes pixel electrodes disposed on the lower substrate. Each pixel is divided into three longitudinally elongated regions of red, green and blue. The directions of the protrusive structures 61 and 62 are bent 90 degrees inside each pixel.
In this structural example, the gap region of the protrusive structures disposed on the upper and lower substrates is divided into four domains A, B, C and D. The alignment directions of the liquid crystal molecules in each domain are mutually different by 90 degrees. In the MVA mode liquid crystal display device, the liquid crystal molecules are aligned in a plurality of directions when TFT is turned OFF as described above. Therefore, the viewing angle can be broadened.
The MVA mode liquid crystal display device involves the problem that its response at a half tone is relatively slow. In the MVA mode, the protrusive structures and the slits formed on the substrate surface regulate the inclining alignment of the liquid crystal molecules. Therefore, the switching operation of the liquid crystal molecules is conducted in the form of a propagation process of inclining alignment of the liquid crystal molecules from near the structures to the gap portions.
FIG. 5 shows the mode of propagation of the inclination of the liquid crystal molecules having negative dielectric anisotropy. Reference numeral 6 in the drawing denotes the protrusive structure, and the other reference numerals have the same meaning as those shown in FIG. 3. The upper part of this drawing shows the initial alignment condition of the liquid crystal molecules. The lower part of this drawing shows the state after the passage of a predetermined time from the timing at which a voltage is applied step-wise to the members 21 and 22.
At first, the liquid crystal molecules near the protrusive structure are subjected to the influences of the voltage change, and their tilt angle becomes great. Because of this change, the liquid crystal molecules that do not keep contact with the protrusive structure and exist in the proximity of the inclined liquid crystal molecules are turned down, and this inclination sequentially propagates. Solid line in the drawing represents the inclination state at the final stage. Dotted line represents the half tone having a small application voltage. Since the force of regulation by the electric field is small, too, a long time is necessary before propagation to the liquid crystal molecules of the whole region is complete. This is the greatest reason why the response of the MVA mode at the half tone is slow.